A filter is a circuit device that passes frequencies within a certain range (a pass band) and rejects (attenuates) frequencies outside that range (a stop band). The attenuation as a function of frequency is known as the filter transmission function. Bandpass filters can contain resonant circuits that in some configurations determine the frequencies at which the characteristics of the filter transmission function occur. The transmission function may be characterized by a distinct drop in the amplitude transmitted above or below a particular frequency (known as a cutoff). The characteristics may include one or more frequencies at which a high proportion of signal power is transmitted (a peak) or where a particularly low proportion of signal power is transmitted (a zero or null). Although low points are conventionally termed zeroes or nulls, the transmission power is typically not zero at such frequencies, which are merely the frequencies at which the attenuation is greatest.
In one example, such as a Chebyshev low pass filter comprising passive circuit elements, a basic form might have one LC resonant element (with just one inductor) that determines a cutoff frequency above which signal power is attenuated. Another example is a bandpass filter with two LC resonators that generate spaced transmission zeros. A bandpass or bandstop filter topology can be provided, for example, by providing highpass and/or lowpass stages in a sequence, the stages having different cutoff frequencies, because the filter characteristics of successive stages are superimposed on one another (i.e., multiplied).
A two resonator bandpass filter may have poor stop band attenuation at low-frequencies. Such performance might be improved by adding additional filter stages, to further attenuate the stop band or perhaps to improve the sharpness of the cutoff between the pass band and the stop band. If additional transmission zeros and other characteristics and stages with resonant elements are desired, more inductors are needed, consuming additional physical area. In these examples and in other ways, multiple resonant elements, each having at least one inductor and capacitor, contribute to filter transmission functions but consume circuit space.
The resonant elements, sometimes termed LC resonators or tanks, can comprise inductors and capacitors arranged in series and/or parallel according to different configurations. The resonant elements are combined, for example, in a ladder circuit. It is possible to employ a number of functionally combined elements with distinct resonant frequencies. Each may typically affect aspects such as the nature of a characteristic in the filter transmission function (e.g., a peak, null or cutoff) and the break frequency at which the characteristic occurs. Each resonant element has at least one inductor. If filter elements are arranged successively in cascade, for example, the transmission functions of the respective elements multiply in determining the transmission function of the cascaded elements.
In integrated circuits, an inductor can be embodied as a conductor arranged a spatial pattern on a substrate, such as the spiral pattern of a so-called Balun coil. However such inductors consume a considerable physical area on the circuit, and space is at a premium. It would be desirable to provide for a bandpass/bandstop function determined by multiple resonant elements that are cooperatively arranged, wherein the resonant elements individually or in cascade or other combination can produce highpass, lowpass, bandpass, periodic (e.g., comb filter) and/or other similar characteristics, but wherein the space required is reduced, compared to the space that would have been needed if a inductor was provided for each resonantly determined characteristic. It is desirable to provide for more complicated configurations with plural resonances and nulls or zeroes, in order to achieve good stop band attenuation, sharp cutoffs and the like, using less integrated circuit space.
Desirable in the art is an improved bandpass filter design that would improve upon the conventional bandpass filter designs by allowing for a large number of frequency breaks using minimized circuit area.